1. Field of the Invention
This invention relates to a developing method and more specifically to a developing method for use in an electrophotographing apparatus or an electrostatic recording apparatus, in which a developing agent is deposited on an electrostatic latent image formed on a photoreceptor or a dielectric body to visualize the image.
2. Description of the Related Art Including Information Disclosed Under .sctn..sctn. 1.97-1.99
In electrophotographing apparatuses and electrostatic recording apparatuses, there has been widely employed, as a developing method providing a good-quality image, a developing method which uses a two-component developing agent consisting of a toner and a carrier to visualize the electrostatic latent image formed on an electrostatic image bearing body such as a photoreceptor or a dielectric body.
This developing method, however, has the following drawbacks.
(1) The toner is charged by mutual friction with the carrier. However, if used for long periods of time, the carrier surface will be contaminated by the toner and the toner will become unable to receive a sufficient amount of charge.
(2) To obtain a desired developing agent, it is necessary to control the mixing ratio of toner and carrier to stay in a specified range. If a developing agent is used for a long time, however, the toner/ carrier mixing ratio deviates from a specified range, making good developing impossible.
(3) Generally, hard particles such as iron powder, the surfaces of the particles of which are oxidized and glass beads are widely used as a carrier. Consequently, the surface of a photoreceptor suffers mechanical damage by the carrier, resulting in the life of the photoreceptor being shortened.
To overcome the problems stated above, various developing methods have been proposed that use a one-component developing agent consisting only of a toner without containing a carrier. Above all, there have been proposed many developing methods that use a magnetic toner containing magnetic powder.
However, the developing methods that use magnetic powder have the disadvantages that follow.
(1) Since the resistivity of a magnetic toner is relatively low, it is difficult to electrostatically transfer a toner image obtained by developing onto a common paper. Particularly in a highly humid atmosphere, the toner cannot maintain a sufficient amount of charge and therefore, satisfactory transferring of the image cannot be performed.
(2) The toner generally contains a large quantity of a magnetic toner consisting of blackish magnetite. As a result, color toners other than those of dark colors cannot be obtained.
For the above-mentioned reasons, developing methods have recently been proposed that use a one-component developing agent which consists of a toner such as used in a two-component developing agent, does not contain magnetic powder, and thus has a high resistivity. Among the developing methods using a one-component developing agent of this type are those which are based on the touch-down method, impression method and jumping method that have been disclosed in U.S. Pat. Nos. 2,895,847 and 3,152,012 and Japanese Patent Publication Nos. 66-9475, 70-2877 and 79-3624.
If the toner used in a two-component developing agent is used as a toner which constitutes a one-component developing agent, this will give rise to problems as follows.
(1) The amount of charge of the toner given by friction with a toner-carrying body is insufficient. Generally, in a developing method using a one-component developing agent, the toner needs to be charged efficiently and in a very short time by friction with the toner-carrying body and obtain a sufficient amount of charge (about -0.5 to -15 .mu.c/g for a photoreceptor made of selenium, for example) to visualize an electrostatic latent image formed on the photoreceptor or dielectric body without physical contact between the toner-carrying body and the electrostatic latent image.
However, with one-component developing agents consisting of a toner used in the conventional two-component developing agents, a sufficient amount of charge cannot be obtained by friction with the toner-carrying agent. More specifically, in the conventional developing method using two-component developing agent, a sufficient time is spent for frictional electrification between the toner and the carrier. On the other hand, in developing methods using a one-component developing agent, the time for frictional electrification between the toner and the toner-carrying body is short, with the result that the amount of charge large enough to visualize the electrostatic image cannot be obtained.
(2) The surface of the toner-carrying body needs to be coated with a toner in a very thin and uniform layer. With the type of toner used in two-component developing agents, it is difficult to form such a thin layer.
With reference to FIG. 1, an example of the formation of a thin toner layer on the surface of toner-carrying body 1 will now be described. As shown in FIG. 1, elastic blade 2 is pressed to the surface of toner-carrying body 1 with a pressure of 20 to 500 g/cm. As a result, while transferred by the rotating toner-carrying body, the toner from toner-container 3 is deposited, by means of elastic blade 2 on the surface of the toner-carrying body in a very thin and uniform layer. Therefore, the toner is required to have sufficient fluidity and anticoagulating property.
The problem with the employment of a toner used in a two-component developing agent for a one-component developing agent is that the toner, if not sufficiently charged, coagulates into lamps as it is carried by revolution of toner-carrying body 1 and cannot be deposited properly on the surface of toner-carrying body 1.
If the pressure of elastic blade 2 is raised in order to increase the amount of charge, toner 4 is suddenly subjected to a high pressure where elastic blade 2 contacts toner-carrying body 1. The resulting frictional heat softens toner 4, which thus sticks to the surface of toner-carrying body 1 instead of being deposited in a thin and uniform layer. If the softening point is raised, the fixing temperature of the toner increases, often causing troubles when such a toner is used in an ordinary electrophotographic copying machine or printer.
As set forth above, by the conventional developing method using a one-component developing agent, the developing agent cannot be sufficiently charged frictionally nor can it be deposited in a thin and uniform layer on the surface of toner-carrying body 1. This results in a low density of the image or fogging, making it impossible to obtain a clear image.